There are several well known techniques for receiving amplitude modulated (AM) signals, and in particular, signals modulated by amplitude shift keying a carrier signal. In such AM signals, variations in the magnitude represent digital data, the digital data of which may represent information to be transmitted. Amplitude modulation of digital data is used in a variety of applications including, but not limited to: pagers, radio watches, radio frequency identification (RFID) tags, fiber optics, and cable modems.
One well known technique for detecting the signal envelope of an AM signal is shown in FIG. 1. A conventional envelope detector 100 may include one or more diodes configured to rectify the AM signal (i.e., diode-based signal envelope detection). Antenna 5 is configured to receive a low frequency broadcast AM signal. For radio clock/watch applications, the frequency of the broadcast AM signal is typically between 40 kHz to 100 kHz. The antenna 5 is coupled to a front end 10 which may be configured to amplify the broadcast AM signal. In addition, front end 10 may also be configured to convert the broadcast AM signal to an intermediate frequency in a heterodyne receiver architecture.
FIG. 2A shows a conventional front end 110 suitable for direct conversion radio receivers (i.e., where the broadcast AM signal is directly converted to a baseband signal). Front end 110 includes an amplifier 120 configured to produce an amplified version of the broadcast AM signal received by antenna 105. Alternatively, and as shown in FIG. 2B, conventional front end 110′ is configured for heterodyne reception (i.e., where the broadcast AM signal is converted to an intermediate frequency before subsequent conversation to a baseband signal). Front end 110′ includes a first amplifier 120′ configured to amplify the broadcast AM signal and provide a first input to a mixer 130. A local oscillator 140 provides a second input to mixer 130. Mixer 130 generally combines the two input signals (i.e., multiplies the amplified broadcast AM signal by the output signal of the local oscillator 140) and provides an input to a filter 150. Filter 150 generally filters one or more spectral components of the output signal of mixer 130. A second amplifier 160 receives the filtered signal and provides an intermediate frequency output signal.
Referring back to FIG. 1, the conventional envelope detector 100 includes a buffer 20 for isolating the front end 10 from subsequent circuitry. Diode 30 provides a rectified version of the output signal of buffer 30 to filter 40. Filter 40 receives the rectified output signal of front end 10. In one example, filter 40 can be a low pass filter with a cutoff frequency less than the carrier frequency and greater than the maximum envelope frequency of the AM signal. The output signal ENVELOPE of filter 40 is a voltage shifted version (i.e., non-zero DC value) of the envelope of the AM signal.
Buffer 50, diode 53, and capacitor 57 may form a peak maximum detector. Similarly, buffer 60, diode 63, and capacitor 67 form a peak minimum detector. Capacitor 57 can be charged to a voltage signal MAX representative of the peak maximum of signal ENVELOPE, and capacitor 67 can charged to a voltage signal MIN representative of the peak minimum of signal ENVELOPE. A resistor divider circuit, which includes resistor 59 and resistor 69, produces a midpoint voltage signal MIDPOINT which is approximately equal to the average of the voltage signals MAX and MIN. The capacitance value of capacitor 57 and the resistance value of resistor 59 must be selected such that the time constant of the RC circuit formed by capacitor 57 and resistor 59 is much less than the minimum frequency of the envelope of the AM signal. Similarly, the capacitance value of capacitor 67 and resistance value of resistor 69 must be selected such that the time constant of the RC circuit is much less than the minimum frequency of the envelope of the AM signal. Differential amplifier 80 can be configured to subtract the midpoint voltage MIDPOINT from the signal ENVELOPE to produce a signal OUTPUT representative of the envelope of the AM signal having a near-zero DC offset.
The output of the conventional envelope detector 100 continuously, and nearly linearly, follows the envelope of the AM signal. In addition, the conventional envelope detector is generally considered to be a simple and low cost solution because it uses passive components, such as diodes 30, 53 and 63, resistors 59 and 69, and capacitors 57 and 67. The conventional envelope detector 100 is generally useful in applications that receive analog-modulated AM signals (i.e., audio receivers).
However, many low-power portable applications receive digitally-modulated AM signals (i.e., radio clocks/watches and pagers) where the detected envelope signal may require subsequent processing to recover the modulated digital signal. The subsequent processing can be achieved by the use of low voltage digital components (e.g., low voltage CMOS microprocessors, microcontrollers, and/or transistors). Modern low voltage CMOS technologies can operate on supply voltages of 1.0 V or less and have typical CMOS transistor thresholds of 0.4 V. But since the voltage drop across a silicon diode (e.g., diode 30 of the conventional envelope detector 100 as shown in FIG. 1) is typically 0.6 V, little or no signal will be received by a typical CMOS transistor. This is because a 1.0 V maximum received signal (i.e., the supply voltage maximum) less 0.6 V typical drop across a silicon diode will have a 0.4 V maximum amplitude at the input to the CMOS transistor (which is or is close to a typical CMOS transistor threshold).
Therefore, a need exists for a low cost envelope detector, operable using very low supply voltages, which is compatible with modern low voltage CMOS technologies.